Perhaps a better way of putting it would be "Which European country is very similar to Benin in both shape and size?"
I think what's actually being asked for, to use mathematical terminology, is which country's shape is closest to being congruent to Benin's (whereas if size were not mentioned, it would be "closest to being similar" instead).
It's a dumb question any way you frame it, but my impression from the questions is that the quizmaker was really struggling to come up with questions that people could get right. Having said that, I still did terribly.
I think what's actually being asked for, to use mathematical terminology, is which country's shape is closest to being congruent to Benin's (whereas if size were not mentioned, it would be "closest to being similar" instead).